Crosslinked polymers can generally be characterised as a network of polymer chains in which at least some of the chains are connected through a bridging group. The nature of the bridging can vary significantly, as can the manner in which the bridging group forms its connection with the polymer chains. The number of bridging groups present, that is the degree of crosslinking, in a crosslinked polymer can also vary significantly, with the effective molecular weight, viscosity and solubility of the polymer changing as the degree of crosslinking in the polymer increases. Polymers with any significant degree of crosslinking are generally substantially insoluble in solvents.
Crosslinked polymers typically exhibit superior physical and mechanical properties compared to their non-crosslinked counterparts. The properties of polymer-based coatings and adhesives (i.e. paints, adhesives, fillers, primers and sealants etc) can therefore also generally be improved by providing such products with a crosslinked polymeric structure. However, due to their crosslinked polymer sure, crosslinked polymers are generally not capable of being applied to a substrate in a manner that is required for the application of most coatings and adhesives. In particular, crosslinked polymers cannot generally be moulded into a desired shape or applied as a layer onto the surface of a substrate.
To provide polymer-based coatings and adhesives with a crosslinked structure and an ability to be readily applied to a substrate, the products are often formulated such that crosslinking occurs after the product has been applied to the substrate. One of the more common formulating techniques used to achieve such post-application crosslinking is to provide the product with at least one polymer which contains reactive functional groups. The reactive functional groups afford sites that can react and promote crosslinking post-application of the product.
One approach to providing such products with these reactive functional groups has been to formulate them with unsaturated natural oils (eg. glyceride oils), or alkyd resins formed therefrom. Compositions formulated with these materials make up a large percentage of coatings used globally and axe commonly referred to as air-dry enamels or oil based paints. Drying or curing of such paints essentially results from the reaction of atmospheric oxygen with the ethylenically unsaturated groups derived from the oils, which in turn promotes crosslinking of the composition in a process known as autoxidation.
However, despite being effective at forming crosslinked polymer structures, the autoxidation process is particularly slow. Sufficient crosslinking necessary to apply a second coat of paint without disturbing the first can only be achieved after the film has been allowed to dry over night. Even then, a range of environmental factors such as temperature and humidity can retard the rate of drying.
The process of autoxidation can also continue for a long period of time (i.e. post drying of the paint) and may result in degradation of the physical properties of the paint film. This degradation can limit the performance of such coatings, particularly in an exterior environment. Oil based paints are also prone to yellowing in the absence of direct sunlight. The tendency for these paints to yellow is believed to stem from a variety of atmospheric based reactions of residual unsaturation derived from the fatty acid segments of the polymer.
An alternative approach to providing such products with these reactive functional groups has been to formulate them with a polymer that contains functional groups that will react with water. In this case, the products can be formulated to form a crosslinked structure after application through being exposed to atmospheric moisture. However, due to their inherent moisture sensitivity, great care needs to be taken to exclude moisture during the manufacture, packaging and storage of such products. Despite exercising care to exclude moisture from the products, moisture cure products often have a limited shelf-life.
Coatings and adhesives are also commonly provided in a two-part form where one part includes a polymer which contains functional groups that are reactive toward functional groups of a polymer contained in the other part. In this case, each part is mixed prior to application, and crosslinking occurs post-application through reaction of the respective functional groups provided from each part. The two-part coating and adhesive formulations have the advantage of being generally less sensitive to moisture and therefore often have a good shelf-life. However, by virtue of their reactivity, the individual components cannot be provided in the form of a single-part composition, as would be most convenient. Furthermore, once the two parts are mixed the product must be used within a relatively short time frame.
Although the aforementioned moisture cure and two-part coating and adhesive products effectively form post-application crosslinked polymeric structures, the reactive functional groups used to provide the crosslinking sites can render the products toxic. For example, reactive functional groups commonly used in such products include isocyanates, amines, epoxides and cyano acrylate esters. Accordingly, there can be occupational health and safety risks associated with both the manufacture and use of such products. Furthermore, the monomers comprising the reactive functional groups used in such products are generally relatively expensive.
Another common formulating technique used to achieve post-application crosslinking is to provide products in a two-part form where one part contains a radical initiator and the other part contains a crosslinkable polymer composition. In this case, the initiator is mixed with the polymer composition prior to application, the mixture is then generally immediately applied to a substrate and crosslinking occurs post-application through a radical mediated crosslinking reaction that is promoted by the initiator. As with the previous formulating techniques, this technique also provides for an effective means to achieve post-application crosslinking. However, such polymer compositions are prone to premature and spontaneous crosslinking, the process of which is very exothermic and potentially explosive. These polymer compositions therefore typically need to be formulated with inhibitors to prevent this. Despite the use of inhibitors, the polymer compositions often have a limited shelf-life. Furthermore, initiators commonly used in these products, such as those which contain a peroxy linkage, are typically quite toxic and potentially explosive in their own right.
Accordingly, there remains a need to provide an alternative method for preparing crosslinked polymers that can overcome or alleviate at least some of the disadvantages associated with the aforementioned methods.